The subject matter disclosed herein relates to impact-protection materials. More specifically, the subject matter disclosed herein relates to structured materials that protect against injury to an individual or damage to a structure resulting from one or more impacts by deforming and thereby limiting peak impact forces experienced by the individual or structure.
Impact protection systems, for example, protective headgear, typically include a relatively hard outer shell and a relatively soft inner liner. In the event of an impact by an object to the outer shell, the shell acts to prevent penetration of the object through the headgear and to distribute the impact load over a larger area. The inner liner acts to limit acceleration of the head by (1) absorbing at least a portion of the kinetic energy of the object via deformation of the inner liner, and (2) by modifying the transmitted impulse profile so as to decrease the peak force.
In many headgear designs, the energy absorbing material of the inner liner is an expanded polystyrene material, and a significant mechanism of energy absorption of such a liner is plastic (unrecoverable) deformation and, under high impact loads, fracture of the material upon impact. Previous plastic deformation (e.g., consolidation) and fracture significantly limits the protective effectiveness of the polystyrene material in the case of repeated impacts.
Other headgear configurations utilize viscoelastic foam inner liners, which remain effective after multiple impacts. Viscoelastic foams provide some degree of protection against blunt impact, but the foam microstructure is “isotropic” and the foam responds in a similar manner when loaded along any direction. This may be disadvantageous when it is considered that loading during impact is typically in one primary direction, i.e., compression orthogonal to the outer hard shell surface. The microstructure of viscoelastic foams is not optimized for this predetermined loading direction and thus viscoelastic foam liners do not exhibit an optimal crush efficiency.
Other protective gear configurations utilize polymeric materials formed into a honeycomb structure, which provides a preferred impact response direction along a cell axis. The cells of the honeycomb typically are hexagonal in shape, with each cell wall shared by two adjacent cells. Often such honeycomb materials are formed from thin sheets of material that are bonded at staggered intervals and expanded to form the honeycomb structure.